1. Field of the Invention
The present invention relates to a sheet feeding apparatus for feeding a sheet separated from a sheet stack one by one.
The sheet is used in office equipment such as copying machines, printers, recording machines and the like and is a cut sheet such as a transfer sheet, photo-sensitive sheet, heat-sensitive sheet, electrostatic recording sheet, printing sheet, original, card, envelope and the like, and is fed to the office equipment. The material of the sheet is not limited to "paper".
2. Related Background Art
Generally, in a sheet feeding apparatus used with the office equipment such as a copying machine, sheets such as transfer papers and photosensitive papers are stacked on sheet support plates disposed within a cassette or a deck, and the sheet is separated from the sheet stack one by one to be fed successively by means of a sheet supply means such as sheet supply rollers.
In order to prevent the double-feeding of the sheets, it is known to provide separating claws or pawls arranged in front of a leading end of the sheet stack in a sheet feeding direction, whereby, when an uppermost sheet is desired to be fed, a bent loop is formed in the front part of the uppermost sheet so that only the uppermost sheet is separated and fed by riding it over the separating claws (separating claw system).
FIG. 31 shows a perspective view of an example of a main portion of the conventional separating claw system.
Referring to FIG. 31, the reference numeral 100 designates sheet support plates on which sheets (cut sheets and the like) P having a predetermined size are stacked; 101 designates bias springs for biasing the sheet support plates upwardly; 102 designates supply rollers; and 103 designates a pair of left and right separating claws engaging the upper surface of the left and right corners of a leading end of the sheet stack P.
The upper surface of the sheet stack P is pressed at its front portion, against the supply rollers 102 at a predetermined urging pressure by the upward bias forces of the bias springs 101 for lifting the sheet support plates. Alternatively, the supply rollers may be lowered to urge the upper surface of the sheet stack P at a predetermined pressure in response to each sheet feed signal.
Each separating claw 103 is pivotably mounted on a pivot 103a and rests on the upper surface of the corresponding front corner of the sheet stack P with its own weight.
When the supply rollers 102 are rotated in the sheet feeding direction, the uppermost sheet P.sub.1 on the sheet stack P is subjected to a feeding force toward the sheet feeding direction by the friction force between it and the supply rollers. Although the uppermost sheet P.sub.1 tends to move forwardly by the feeding force, since the left and right corners of the front end of the sheet P.sub.1 are restrained by the separating claws 103, the sheet P.sub.1 cannot be moved. As a result, as the supply rollers 102 are rotated, a bent loop A is formed in the uppermost sheet P.sub.1 between the separating claws 103 and the supply rollers 102 in opposition to the hardness (of the sheet material) to be bent.
When the bent loop A grows up to a certain level, the left and right :corners of the uppermost sheet P.sub.1 pressed down by the respective separating claws 103 are automatically moved from the undersides of the separating claws to the uppersides thereof to ride over the separating claws by the restoring force of the sheet tending to varnish the loop. In this way, only the uppermost sheet P.sub.1 is released from the restraining force of the separating claws and is separated from the remaining sheet stack P to be fed (as shown by the two-dot chain line).
In such a sheet feeding apparatus having the separating claws system, if the sheets to be separated and fed one by one comprise thicker sheets (having the hardness to be bent stronger than that of a normal or plain sheet) such as postcards, library cards or envelopes, the above-mentioned bent loop A is not or almost not formed in the sheet between the separating claws 103 and the supply rollers 102 because the resistance to bending (stiffness and/or elasticity of the sheet) overcomes the feeding force of the supply rollers 102. Consequently, the uppermost thicker sheet cannot ride over the separating claws, thus causing the impossible or insufficient separation of only the uppermost sheet from the remaining sheet stack. If the stronger feeding force is applied to the thicker sheet to try to separate the latter from the sheet stack, the front corners of the thicker sheet are bent or folded while the latter rides over the separating claws, which would result in the jamming of the sheet or malfunction of the sheet feeding apparatus.
To solve the above-mentioned drawback, sheet feeding apparatuses having retard rollers for handling the thicker sheets nave been proposed. Such apparatuses, however, were very expensive and complicated.